CARE OF LUMBER
=14. The piling of lumber.=--(=A.=) To the uninitiated it may seem that
the piling of lumber is work upon which it is not necessary to expend
much skill, but there are few operations in which carelessness or
ignorance will cause more loss to a wood-worker.
(=B.=) The front end of a lumber pile should be higher than the back,
therefore it is a good plan to locate it upon ground which falls away
to the rear, or to build the ways which support the pile so that the
water which drives into the pile will run out at the back end, and not
stand upon the boards, as this will cause discolorations.
=15. Permanent lumber ways.=--These should be built by some method
similar to that shown in Fig. 22. It is not a good plan to lay timbers
upon the ground, as they will decay rapidly, and there will not be
sufficient room for air to circulate under the pile to allow the boards
of the lower courses to dry out properly. The pile is also apt to
settle when the frost comes out of the ground in the spring. Lumber
should not be stacked above wet or marshy ground; if necessary to stack
it where the weeds are of rank growth, the latter should be kept down.
The ways should be built with a solid foundation, well below the frost
line, though this is rarely done except for permanent lumber storage.
This is shown at _a_, Fig. 22, in which it will be seen that the ways
are built to stand a heavy load; the space between the centers of the
ways should be about five feet, as multiples of this distance will
accommodate any length of boards.
[Illustration: FIG. 22.--PERMANENT LUMBER WAYS.]
=16. To minimize the warping of lumber.=--(=A.=) Do not place lumber
piles less than one foot apart, as it is necessary that there should
be a continuous circulation of air through the pile in all directions.
(See _b_, Fig. 22.)
(=B.=) Lumber piles are usually four feet in width, and should be built
up with sticks of that length, which are placed between the courses of
boards. It is important that these be placed directly over each other
and the ways; otherwise there will be short kinks in the boards, as
shown at _c_. It is such carelessness as this that causes a great deal
of loss. In piling very expensive lumber, the front sticks should be
laid so as to project a little over the course of boards below, and
the boards of the course above should project the same distance over
the stick, in order to give the front of the pile an inclination to
the front, as shown at _d_, which will allow most of the rain-water to
drop clear of the boards below, instead of running down the front and
finding its way into the pile.
(=C.=) Square piles are sometimes built, but in these the boards should
be laid with large spaces between them, to allow perfect circulation
of air. It is obvious that in a pile of this sort, the boards in the
center of the pile will not come in contact with the air as much as
those on the outside, and that consequently, unless carefully piled,
the boards may be damaged by the moisture souring instead of drying
out, which usually results in decay.
(=D.=) During the drying out process, all boards change their form
more or less, depending upon the shape of the tree trunk, the kind
and quality of the wood, the part of the tree from which the log was
cut, as well as its size and age, the relation of the annual rings and
medullary rays to the surfaces of the board, the length of time since
the log was cut before being made into lumber, whether it had lain in
water for several months, and the method of piling. Thus it will be
seen that in every stage of preparing lumber for market, a high degree
of skill and judgment is necessary to insure the best results.
[Illustration: FIG. 23.--WARPING OF LUMBER.]
The greatest deterioration in lumber, after it has been cut and
properly piled, is generally due to the tendency to warp, the cause
of which is indicated in Fig. 23, and which may to great extent be
minimized by skillful piling. If this sketch is studied carefully, it
will be noticed that the middle board is thicker in the middle than
it is at the edges, and that the curves of its top and bottom sides
are practically uniform. This is because the annual layers are at
nearly a right angle with the sides of the board, which causes the
board to shrink in thickness, and very little in width. This is due
to the tendency of lumber shrink around, or parallel with, the annual
layers. This tendency also causes the star shakes, as at _c_, Fig.
3, which is because the inner layers of the log, being less than the
outside layers in circumference, and less exposed to the dry air, do
not shrink so fast nor so much. This tendency is again illustrated in
Fig. 23, in which it will be seen that because the outer annual layers
shrink faster, they cause the outside of the board, or the part which
grew toward the outside of the tree, to become narrower, and to assume
a concave shape, while the side nearer the center of the tree, or the
inside of the board, becomes convex. This is also the reason why boards
cut near the outside of the log will shrink in width more than those
cut nearer the center, which shrink in thickness proportionately more
than in width.
(=E.=) For the purpose of taking advantage of the tendency to warp, and
applying it to its own remedy, boards should be piled with the side
which grew nearer the center of the tree uppermost. This will help to
correct the tendency of the board to warp, as explained above, as the
side which would naturally assume the concave shape will be underneath,
and less likely to warp than if it were uppermost. This is not
generally observed in stacking common lumber, since it needs care and
judgment to do it properly, but it should be done if valuable lumber is
being handled.
[Illustration: FIG. 24.--LUMBER PILED IN DOUBLE COURSES.]
Boards of practically the same width, if less than 7” wide, are
sometimes stacked in double courses, as shown in Fig. 24, the outside
of the boards, or the sides which grew nearer the outside of the tree
being placed together, thus allowing the inside of the boards, or the
sides which grew toward the center of the tree, to receive more air
than the sides which are placed together, and therefore to dry out
faster, which will reduce the warping to a minimum. After a pile is
completed, it should be covered with old boards to protect the top
courses from the weather.
(=F.=) A pile of valuable lumber should be restacked every six or eight
months, as the boards are apt to become discolored where the lumber
sticks are placed; in this rehandling, the warped boards should be
placed with the concave side underneath.
(=G.=) If lumber is cut in winter or midsummer, and properly cared for,
it is not apt to be injured by any rain which may drive into the pile,
if there is free circulation of air; nor is it so liable to decay as
lumber which is cut at other times of the year.
=17. Weather-dried lumber.=--Lumber which has been dried in the stack
out of doors is not dry enough for use in the manufacture of inside
finish or furniture, as it has dried out only to the degree of
moisture in the outside air. If it is then worked up and placed in
an artificially heated house, the heat will cause more moisture to
evaporate, the wood to shrink, and the joints to open. For material to
be used in the frames of buildings, in wagons, or in other places where
the greatest possible strength is required, not less than two years
weather-drying is preferred, as the material retains its full strength.
=18. Kiln-dried lumber.=--Lumber for furniture or for inside finish
should be seasoned by the process known as “kiln-drying.” This means
that lumber is exposed to a temperature of from 120° to 200° F. by
which the moisture is extracted and evaporated. Lumber thus treated
is apt to be more or less weakened by the action of the heat upon the
fibers of the wood, which causes thousands of minute fractures, and in
many cases the life and the elasticity of the lumber is destroyed. The
results of kiln-drying depend largely upon the kiln, and upon the skill
with which the lumber is piled, the heat applied, and the rapidity of
evaporation of the moisture regulated.
For these reasons, much kiln-dried lumber is suitable for use where but
little strength is required and where the color and the grain are the
important points to consider.
=19. Moist air kilns.=--(=A.=) There are two types of dry kilns in
common use: the _natural draft_, or moist air, kilns, and the _induced
draft_ kilns. These two types are made by different manufacturers,
nearly all of whom use certain devices of which they control the
patents, and which constitute the chief difference between their kiln
and those made by other manufacturers.
(=B.=) The moist air kilns are so constructed as to allow the freest
possible circulation of the heated air, and to provide opportunities
for the moisture to be expelled in accordance with certain natural
laws, which results are obtained by a carefully planned and managed
system of ventilation. These kilns operate upon the principle that
heated air circulating naturally through lumber will become charged
to a much greater degree with moisture than if it were forced through
rapidly, as in the induced draft kilns. Thus, heated air by passing
slowly through a pile of lumber may become charged with moisture nearly
to the dew point.
If the humidity of the heated air is maintained at that point, by
allowing the moisture to pass out as it accumulates, with a small
amount of heated air, which is replaced with fresh air from the
outside, it is claimed that the boards will dry out from their centers.
(=C.=) As the warm, moist air which circulates through the pile will
keep the outsides of the boards moist, it will prevent case hardening,
or the hardening of the outsides of the boards. This is caused by
very warm dry air, which “cooks,” or closes the pores of the surface
of the boards, and this prevents the outsides from shrinking, while
the insides will be so badly checked and discolored as to destroy the
boards.
After the moisture is all out of the lumber, that held in suspension
will gradually pass out of the kiln, and the air inside will become
perfectly dry.
(=D.=) It is claimed that all kinds of lumber in common use may be
put into this type of kiln perfectly green, except oak and other very
hard woods, which should have at least thirty days’ drying under good
drying conditions for each inch in thickness. It is also claimed that
the moist air kiln is simply weather drying accelerated,--the moisture
being thoroughly extracted from the lumber, the result being the same
as though it were stacked out of doors for several years,--and that the
lumber has lost none of its strength, elasticity, or characteristic
color.
(=E.=) This method sometimes is applied by steam pipes extending
between each course of boards, and in this way the lumber is dried out
very rapidly. Lumber used in this sort of kiln should be thoroughly
weather-dried, or otherwise the high temperature will cause it to check
badly. In certain forms of these kilns, the lumber is saturated with
live steam after it is piled in the kiln, before the heat is turned on.
=20. Induced draft kilns.=--(=A.=) This system of kiln-drying consists
of a power-driven fan, which forces the heated air at a high rate of
speed through the spaces between and around the lumber piled in the
chamber.
(=B.=) Manufacturers have different devices for extracting the moisture
from the air after it has passed through the lumber piles. It may be
passed over condensing plates, or through coils of pipes in which
cold water is continually circulating, both of these devices being
for the purpose of extracting the moisture from the heated air. If
the moisture is separated from the air by condensation, it runs away,
but if not, a certain per cent of the heated air is expelled out of
doors, being replaced by fresh air. The air in the kiln, somewhat
cooled from contact with these cooled surfaces, is returned to the
heater, reheated, and again forced through the kiln, which operation is
repeated continuously and automatically. Thus the heated air becomes
charged with a small percentage of moisture each time it passes through
the kiln chamber; this moisture is extracted and the air is again
heated before beginning another circuit, instead of slow circulation
which allows the heated air to become saturated with moisture before it
is discharged, as in the moist air kiln.
The induced draft dry kiln requires quite an expensive equipment, as
the blower and the appliance which drive it are necessary in addition
to the equipment of the kiln itself, which would be similar in either
of the types of kiln described. Lumber to be dried in this form of kiln
must be well weather-dried before it is exposed to the high temperature
of the kiln.
=21. Results of the two systems.=--While it is not the province of
this book to pass judgment upon the results of the different methods
or forms of dry kilns, it is obvious that the induced draft kiln is
the more expensive to operate, as the expense of running the blower is
avoided in the moist air system. In this latter type of kiln the steam
simply passes through the pipes, the condensation being returned to the
boiler to be reheated, so the only expense is that of maintaining the
fire to keep up a low pressure. In the daytime, or while the engine
which furnishes the power for the plant is running, the kilns of either
type may be heated by exhaust steam.
Many users of one or both systems seem satisfied with the results
obtained from either, while others are decided in their preference.
=22. Filling a kiln.=--In doing this, care should be used that there is
plenty of room for the air to circulate freely around and through the
pile--not less than 3” between the edges of the boards horizontally and
vertically, and one foot between the lumber and the wall or adjacent
pile. Each course of boards should be so planned as to bring the same
width over those of the course below, if possible, in order to keep a
vertical air space through the pile. In some cases the kiln is filled
by placing the boards edgeways.
=23. Length of time lumber should be left in the kiln.=--No one should
undertake to operate a kiln unless he understands perfectly the
particular make of the kiln that he is handling, for if the ventilation
is not correctly regulated, the entire charge of the kiln may become
mildewed, casehardened, checked, discolored, or dried unevenly. No rule
can be given for the time which lumber should be left in the kiln,
as it depends upon the condition of the lumber, temperature, kind of
lumber, dimensions, and ventilation. Generally speaking, if the kiln
is properly constructed and operated, from two to four days for each
inch in thickness of soft wood, and from two to three times as long,
at a lower temperature, for hard wood, is usually enough to extract
the moisture. It is, however, best to allow the lumber to stay in the
kiln, at a moderate temperature, from three days to two weeks after
the moisture is extracted, in order to harden and cook the solids of
the sap, as by so doing the lumber is not so liable to be influenced
by moisture in the future; this is the effect that long weather-drying
accomplishes.
=24. The care of kiln-dried lumber.=--It is a common mistake to allow
lumber to lie in an open shed or other place where it will absorb
moisture from the atmosphere, and still call it kiln-dried. Lumber
of this sort should be kept in a place where heat can be applied in
damp weather, and should be stacked in a close, compact pile, so as to
prevent the air from coming in contact with it.
=25. Steaming wood.=--This process makes wood pliable, and adds to
its durability by destroying the germs which may cause decay; it also
neutralizes, to a great extent, the effect of the presence of sap.
Steaming or immersing wood in boiling water minimizes its tendency
to shrink and swell, and wood thus treated is not so apt to check in
seasoning. Steamed wood loses some of its original strength on account
of the effect of the high temperature upon the fibers.
=26. Preserving wood.=--In order to preserve wood, it is sometimes
treated with creosote or other chemicals, which are forced into the
wood at a sufficient pressure to cause them to permeate the wood
thoroughly. This treatment enables the wood to resist better the
elements and to keep away insects, which do a great deal of damage,
frequently honeycombing the wood with holes, with little or no evidence
of their presence upon the outside.
SUGGESTIVE EXERCISES
14. What are some of the results of piling lumber carelessly? Should
the back and the front of the lumber pile be upon the same level? Why?
15. How should lumber ways be built? What kind of places should be
avoided in seeking a location for lumber piles?
16. Should the piles be placed close to each other? How wide should
the piles be made? What is the objection to a square pile? How thick
should the lumber sticks be? How should they be placed? What is the
result if they are not carefully placed? How should the sticks and
the ends of the boards be placed at the front of the pile? Why? What
causes lumber to warp? Describe methods of piling lumber to minimize
warping. Should a lumber pile be allowed to stand indefinitely? What
is the proper time to cut lumber? Does it injure lumber to allow a
little rain to beat into the pile?
17. What is meant by weather-dried lumber? Why is it not suitable
for furniture and for inside finish? How is this remedied? For what
purposes is weather-dried lumber the best?
18. What is the chief objection to kiln-drying lumber?
19. What are the two methods of kiln-drying? Describe the principle of
the moist air kiln. What is claimed of it? How should hard wood lumber
be treated before being kiln-dried?
20. Describe the induced draft system. What devices are used to
extract the moisture from the heated air? What are the main points of
difference between the two systems?
21. What is the difference in the condition of lumber which may be
put in the two forms of kilns? Which is the more expensive system to
install and operate? How do users of the two systems compare them?
22. How should lumber be stacked in the kiln?
23. How long should lumber generally remain in the kiln to allow the
moisture to be extracted? How long to insure most permanent results?
24. How should kiln-dried lumber be cared for?
25. What is the effect of steaming wood?
26. How is wood sometimes treated to preserve it from the elements and
from insects?